What Drove Homo Erectus Out of Africa?

A vast expanse of thistles and dry grasses stretches out into the distance on a hot summer day at 'Ubeidiya in northern Israel. The mountains of Jordan can be seen in the distance. Nearby are cultivated olive groves as well as a date palm plantation.
Just south of the Sea of Galilee and along a rocky dirt road is 'Ubeidiya. There are no signs to indicate its archaeological riches. According to Omry Barzilai of the Israel Antiquities Authority as he walks through hillside brambles, 'Ubeidiya's panorama might have been quite different 1.5 million years ago. He says, "You would have seen large lakes that stretch all the way to Jordanian hills."

Hippopotami would have enjoyed aquatic plants. The landscape was densely covered in wild oak, olive and pistachio tree trees. One might also have seen some modern human relatives on the lakeshore: a group of Homo Erectus using sharpened stone handaxes to remove the carcass of a deer, or hippo, which was then killed by a saber toothed tiger.

H. erectus, sometimes called Homo Ergaster, settled 'Ubeidiya as a stop on his journey out of Africa. This ancient site, named after a nearby Palestinian Arab village that was discovered by local farmers collective Kibbutz Alfikim in 1959, may be the key to understanding how H. erectus moved from his home.

H. was pushed or pulled by what? It is still a controversial issue to determine what erectus means in Africa.

Did it come down to innate adaptability such as curiosity, social learning, or a love for meat? Was it the rapid climate change or expansion of grasslands that sent them on their way? Or was it a combination of all these factors?

There are many questions about resilience and innovation at stake. Miriam Belmaker, a paleoanthropologist at the University of Tulsa, Oklahoma, states that we must "change the question from the where, what and when' of dispersal to the why it was successful."

H. erectus is a fascinating species. It's best known for its unique "firsts". H. erectus was taller than any of its predecessors and had shorter arms and legs.

They expanded over the course of 1.75 million years into Western Asia, then into Eastern Asia including what is now China and Indonesia. Andy Herries, a paleoanthropologist at La Trobe University, Melbourne, Australia, says H. erectus (which means "upright man") is also considered to have been "the first runner."

Herries states that "Homo Erectus is one the most successful human races to have ever existed." "It appears to have evolved around 2 million years ago and, if the latest dates from Indonesia are accurate, it was still about 108,000 years ago." Herries believes modern humans may have lived on Earth for a shorter time due to climate change. "Homo sapiens can only dream of such a long existence at 300,000.

H. erectus was also a first hominin, meaning that he belonged to our ancestral human line. He made two-sided, teardrop-shaped, stone hand axes. These are known as Acheulean Tools. The oldest of these tools dates back to 1.7 million years. Most scholars believe that H.erectus evolved in Eastern Africa's Rift valley because of the abundance of fossils and tools found there. But a 2.04-million-year-old cranium, found by Herries in South Africa, indicates that these hominins were on the move 2 million years ago. H. erectus' incredible migrations from Africa to Africa are remarkable and eventually allowed the species to live in half of the world.

H. erectus crossed the Levantine Corridor on their journey from Africa to Europe, Asia and Africa. This narrow strip of land between the Mediterranean Sea and the desert to its east includes the present-day countries of Syria, Lebanon and Israel as well as Palestine, Palestine and Jordan. H. erectus visited the 'Ubeidiya location between 1.2 million and 1.6million years ago. It was a station along his route through Levant.

Scholars have subscribed to the "Savannahstan” hypothesis for many years to explain hominin migrations from Africa. This theory states that H. erectus was dispersed from East Africa around 2 million years ago, when climate change caused the expansion of East African Savanna into the Southern Levant.

H. erectus probably stayed close to water sources, lakes and rivers, during their long journeys, according to Bienvenido Martinez Navarro, a paleontologist at the Institut Catala de Paleoecologia Humana i Evolucio Social, Tarragona. They likely ate the meat of dead animals as scavengers. The original Savannahstan hypothesis states that these hominins are so adaptable to open grasslands and occasional patches of woods, that they were able to follow new savannas from Africa as the climate changes.

However, the discovery at 'Ubeidiya has complicated the idea that H. erectus was passively following the spread of savanna. Belmaker reminds us that 'Ubeidiya was not a savanna. It was a forest, covered with trees, which her work helped to establish.

The evidence for Belmaker's position can be found in the hundreds of thousands of fossilized bones of animals that were excavated at 'Ubeidiya. They are mostly kept in drawers at National Natural History Collections at Hebrew University of Jerusalem. They include the teeth and bones from rhinoceroses and crocodiles as well as hippopotamis, crocodiles and bears. The 6-foot-wide horns, which measure 6 feet in width, of an extinct buffalo species are located at Jerusalem's Israel Museum.

Belmaker explains that most hoofed mammals at 'Ubeidiya came from Eurasian countries, including deer and elk. This indicates that the area was not an African savanna. Extinct horses, deer, giraffes and wild cattle have wear patterns on their molars that suggest these ungulates ate soft leaves more typical of woodland vegetation than grassland.

Belmaker also compared the compositions of four Eurasian hominin sites and the remains of four carnivores from 'Ubeidiya (between 2.6 and 770,000 years ago). Two of the H.. erectus sites, Pirro Nord, in Italy, and Dmanisi, in Georgia, were inhabited by a variety of animals. These included hyenas, dogs, and other species that prefer open, long-distance running environments. This suggests that these sites may have been shrub or grassland. Three other sites, namely 'Ubeidiya and Venta Micena (Spanish) and Akhalkalaki (Georgia), had a wider range of ambush hunters than the two in Georgia. This suggests that they were forested.

Belmaker's research suggests that H. erectus can thrive in multiple habitat types and is not restricted to savannas. She has a new theory for migration based on her findings. H. erectus was naturally adapted to different landscapes, even before other groups left Africa. This includes hunting antelope in open plains, and clearing forest patches.

Belmaker's hypothesis is supported by evidence from Eastern Africa that H. erectus was well equipped to thrive in all kinds of habitats. Isabelle Winder (zoologist at Bangor University, Wales) has found that the African Rift Valley where H. erectus was discovered would have been characterized by "rough" or irregular landscapes rich in caves and basins that held water and sediments.

These spaces provided hominins with places to hide from predators and forage. However, the difficulties associated with these habitats meant that individuals with adaptations that increase their survival in multiple habitats (such as feet that act like levers to help hominins climb over boulders) would be most likely to survive and reproduce. H. erectus could therefore have evolved to be more versatile over time.

Winder states that such complex terrain can also be found along coastlines and would have created "plausible routes" from Africa, which could have facilitated early Homo's expansion. These diverse landscapes included hills, valleys and patches of forest, water and diverse vegetation.

Belmaker also believes H. erectus could have had adaptations that went beyond his physical capabilities. She claims that H. erectus was unique because they were either biologically smarter or had a social structure that allowed them to succeed in such novel environments.

Belmaker refers to a skull that belonged to H. erectus' ancestor from the 1.77million-year-old Dmanisi Site in Georgia as support. The bones are likely to have been left behind by a man who had lost his teeth in the years before his death. Although there are many possible scenarios, Belmaker suggests that this hominin survived because other people cared for him. They helped with the hard work involved in gathering, hunting and preparing root vegetables and raw meat.

These ideas are a radical reimagining of the capabilities of ancient hominins. Belmaker concludes that "Homo Erectus was not passive in its environment." It didn't follow the flow, 'Oh, more grassland. I'll go here', but was an active participant in its own destiny. It means they were able to choose to live in forests, which is a sign that they have some agency over their destiny.

Others agree with H. erectus' ability to adapt to different environments.

"The evolution of human history has seen a rapid increase in different abilities to occupy various environments," Rick Potts, paleoanthropologist at the Smithsonian Institution, says. "This includes eating a greater range of foods and being able cognitively and socially to adapt to a wider range of situations."

He points out that H. erectus was present in tropical Southeast Asia and Indonesia around 1.4 million to 1.6million years ago. "This is also an indication that not only one habitat is being used, but many are being observed."

Potts however believes that there was an immediate trigger that triggered adaptations: periods with highly variable climate.

Potts began to think about the possibility that climate variability could be a factor in major evolutionary changes since the 1980s. He believes that only those with certain traits can survive and thrive in times of sustained and rapid climate change. This will allow them to raise children who, in turn, can pass on the beneficial traits and shape human evolution.

Cognitive abilities, such as the ability to create sophisticated stone tools, could have enabled people to eat different foods in different environments. A trait such as curiosity could have encouraged hominins move to humider climes after the landscape dried.

Potts emphasizes that "Homo Erectus didn’t have a map." They didn't realize they were outside of Africa. They were simply going to the next valley to look for what was there." Their traversal of many hills and valleys over generations would have resulted in dispersal.

Potts published a paper in 2015 in the Journal of Human Evolution. He looked at several species of hominins to see if there were signs that climate variability favors the development of beneficial traits. Together with anthropologist Tyler Faith, now at the University of Utah, the pair mapped periods of high and low climate variability for tropical Eastern Africa over the past 5 million years, specifically looking at once-every-100,000-year shifts in the Earth's orbit that prompt more frequent switches between periods of drought and high rainfall. Faith and Potts found that high levels of climate variability coincided closely with important milestones such as the emergence and development of advanced stone technology, migration, brain growth, and bipedal Australopithecines.

Numerous major milestones in hominin history, such as the dispersals H. erectus or H. sapiens, occurred during periods of high, prolonged climate variability. Potts said that the pattern was so obvious, it looked rigged.

What could climate variability have done to H. erectus's development? Peter de Menocal is a marine geologist and climate scientist. He has used layers of sediment from the East African coast to study climate changes 1.9 million years ago. He points out that the "period of about 2 million years [ago] was one of the most important junctures of human evolution."

De Menocal says that there were many adaptations, such as the slimmer bodies and longer legs, which gave H. erectus an increased ability to run or walk long distances. He believes that more meat was available on the savanna to support their increased energy needs. The increase in brain function presumably led to "a greater ability to plan, to coordinate and communicate."

The bifacial Acheulean Axes are a significant innovation in H. erectus that first appeared in the fossil record 1.76million years ago. They were found at Kokiselei, near Lake Turkana in Kenya. These axes are far more advanced and sophisticated than any of the earlier hominin tools. This tool may have been supported by cognitive and physical adaptations. Rachel Lupien (postdoctoral researcher scientist at Columbia University's Lamont-Doherty Earth Observatory) used chemical signatures in plant waxes preserved in Turkana Basin sediments to detect a shift in climate that roughly corresponds to the rise in this technology.

Lupien states that Acheulean handaxes were better suited to rapid environmental changes, as they were more multipurpose tools. This allowed H. erectus thrive in different environments.

Lupien believes that H. erectus could have dispersed from Africa because of its natural adaptability and rapid climate changes. She emphasizes that climate variability played a significant part in H. erectus' dispersal out of Africa. "I have seen very large climate swings coincide with the most current dates on these transitions. That coincidence, I believe, is not random."

Some scholars still believe in a Savannahstan hypothesis variation, which presents savanna to be a mixture of grassland with some woodland. Many people are still debating H. erectus’ journeys out West Africa. The question is not whether H. erectus was adaptable in different landscapes but what drove this hominin’s flexibility.

Belmaker acknowledges that Potts hypothesis has "nice" correlations between climate changes and the emergence new adaptations. She says that each generation of H.erectus would have seen the climate as relatively constant after a long period of climate variability. She believes that climate change wouldn't have had a significant impact on the passing of traits from one generation into the next.

Belmaker also argues that many milestones that Potts considers to be related to climate change were not present at all. Stone tools can be traced back to 3.3 million years ago, long before H. erectus arrived on the scene.

Belmaker believes that this hominin's ancestral DNA included adaptability. She believes they were successful because they were socially connected and generalists.

Others, such as de Menocal, are convinced that "the making" of humans is fundamentally linked to environmental changes. This is especially important as H. sapiens faces its own, human-caused climate change.

"We are smart. De Menocal states that in our best selves we are adaptable to change and make smart decisions based upon the existential threats these changes present to us. "Our history has been one of adapting to big changes. The larger message is that we must pay attention to threats.

This story originally appeared in Sapiens, an anthropology journal.

Josie Glausiusz, a science journalist from Israel, is the author of this article.